Two-color differential display as a method for detecting regulated genes

ABSTRACT

The invention relates to a novel method for analyzing the composition of an mRNA sample and analyzing differential gene expression using two differently labeled primers, and to the use of the method.  
     In the method for analyzing an RNA sample,  
     a) a first primer, which is, where appropriate, labeled with a first dye, is used to prepare the first strand of a complementary DNA sample (cDNA sample) from an RNA sample,  
     b) a second primer, which is preferably labeled with a second dye, is used to prepare the second strand of this cDNA sample,  
     c) the first primer, which is labeled with a first dye, and the second primer, which is labeled with a second dye, are used to amplify the cDNA sample, and  
     d) the composition of the amplified, labeled cDNA sample is analyzed.

[0001] The invention relates to a novel method for analyzing the composition of an mRNA sample and analyzing differential gene expression using two differently labeled primers, and to the use of the method.

[0002] Differential RNA display (DD) is one of the methods which is most frequently used for detecting and isolating regulated genes (Liang, P. and Pardee, A. B. (1992) Science 257, 967-971; Liang and Pardee (1995) Current Opinion in Immunology 7, 274-280; McClelland et al. (1995) Trends in Genetics 11: 242-246). The DDRT (differential display+reverse transcription: DDRT) method involves, in a first step, reverse transcribing isolated RNA using a first reverse primer (reverse transcription (RT)), thereby preparing the first strand of a complementary DNA (cDNA), then amplifying this DNA, in a second step, by the polymerase chain reaction (PCR) using the first primer and a second primer, and then analyzing the composition of the amplified cDNA sample, for example by fractionating the amplified cDNA in a gel. While detection can be effected, for example, by hybridizing with a labeled probe or by labeling the amplified cDNA, it can also be effected by carrying out the amplification in the presence of radioactively labeled nucleotides (usually radioactively labeled dATP) or in the presence of a labeled primer which is, for example, labeled with a fluorescence (“fluorescence DDRT”) (Ito et al. (1994) FEBS Letters 351, 231-236; Ito and Sakaki (“Methods in Molecular Biology Vol. 85: Differential Display Methods and Protocols” (1997) p. 37-44 Liang and Pardee eds, Human Press Inc. Totowa, N.J.; Jones et al. (1997) Biotechniques 22, 536-543; Smith et al. (1997) Biotechniques 23, 274-279).

[0003] If the reaction is carried out in the presence of radioactively labeled nucleotides, all the amplified cDNAs are then labeled (radioactive DDRT; classical DDRT). If, on the other hand, a labeled primer is used (e.g. fluorescence DDRT), only a part of the amplified cDNAs is labeled, as a result of the possible primer combinations of the first labeled primer and of the unlabeled second primer during the PCR, while another part remains unlabeled.

[0004] The first labeled primer (in this case, primer 1 is an oligo(dT) primer which possesses two further nucleotides (M=A, C, G; N=A, C, G, T) at the 3′ end (“5′-(T)₁₂-MN-3′” is (T)₁₂-MN)) is used for the RT. In the subsequent PCR, a second, unlabeled primer (in this case, primer 2 is an oligonucleotied which has a sequence of 10 randomly ordered nucleotides (“10mer”)) is used in addition to this first primer; however, the first primer is customarily used in a twofold excess. In this way, a relatively high proportion of amplified cDNAs whose 3′ ends exhibit the sequence of the first primer and whose 5′ ends exhibit the sequence of the second primer (e.g. 5′-10mer ------ (T)₁₂-MN-3′, where “------ ” symbolizes the amplified cDNA sequence which is located between the primer sequences) is obtained as a rule. In addition to this, cDNAs are amplified in which the primer sequences are transposed as compared with 1) (at whose 5′ and 3′ ends can be found the sequences of the (T)₁₂-MN primer and of the 10mer primer, respectively; cf. 2)). Other cDNAs are either amplified only using the (T)₁₂-MN primer (cf. 3) or only using the 10mer primer (cf. 4). Consequently, when two different primers are used, it is possible to amplify the following reaction products in a DDRT as a result of the possible primer combinations: 1) 5′-10mer (T)₁₂-MN-3′ 2) 5′-(T)₁₂-MN 10mer-3′ 3) 5′-(T)₁₂-MN (T)₁₂-MN-3′ 4) 4)   5′-10mer 10mer-3′

[0005] Owing to the fact that only the first primer is labeled in this fluorescence DDRT, only those cDNAs which were amplified in at least one direction using the labeled first primer, i.e. the cDNAs specified under 1), 2) and 3), are detected in the subsequent analysis of the amplified cDNAs, whereas cDNAs which are only amplified using the second primer are neither labeled nor detected (cf. 4). Consequently, the complexity of the detected PCR products (cDNAs) is less than that in a radioactive DDRT. For this reason, the conventional fluorescence DDRT does not detect those regulated genes, or the mRNAs which correspond to them, which are only amplified using the second primer. Consequently, the number of regulated genes which is detected by the conventional fluorescence DDRT can be lower than that detected in a radioactive DDRT.

[0006] Furthermore, when one single mRNA is used, a uniform cDNA product is not amplified in the PCR either with conventional radioactive DDRT or with conventional fluorescence DDRT, as shown under 1) to 4); instead, several cDNAs, which differ in their length (this would be evident, for example, by fractionating in a gel), can be amplified as a result of the different primer combinations. Only a detailed analysis (e.g. a sequence analysis) would show whether different amplified cDNAs are cDNA products of one and the same regulated gene or not. The conventional DDRTs do not provide the possibility of differentiating between the redundant labeled, amplified cDNAs. In a radioactive DDRT, it is not possible to differentiate between 1) to 4). In a conventional fluorescence DDRT, it is not possible to differentiate between 1), 2) and 3), although there is a greater probability that the visible cDNA fragment derives from the 3′ region of a gene since the 3′ primer is labeled with the fluorescence dye.

[0007] Consequently, the redundancy, on the one hand, of amplified cDNA fragments in conventional radioactive DDRT, which redundancy cannot be established or can only be established with a relatively large amount of effort, and, on the other hand, the lower complexity of the amplified cDNA product in conventional fluorescence DDRT as compared with conventional radioactive DDRT, with the risk of overlooking particular regulated genes, are the main problems of conventional radioactive DDRT and conventional fluorescence DDRT methods, respectively.

[0008] The invention on which the present application is based provides a method which can be used for analyzing RNA samples and in particular for analyzing differentially regulated genes, which method does not suffer from the abovementioned disadvantages.

[0009] The present invention relates to a method for analyzing an RNA sample, preferably an mRNA sample, which comprises

[0010] a) using a first primer, which is, where appropriate, labeled with a first dye, to prepare the first strand of a complementary DNA sample (cDNA sample) from an RNA sample, preferably an mRNA sample,

[0011] b) using a second primer, which is preferably labeled with a second dye, to prepare the second strand of this cDNA sample,

[0012] c) using the first primer, which is labeled with a first dye, and the second primer, which is labeled with a second dye, to amplify the cDNA sample, and

[0013] d) analyzing the composition of the amplified and labeled cDNA sample.

[0014] The RNA sample contains mRNA to be analyzed. In a preferred embodiment of the invention, mRNA is used in the method. The RNA can be isolated, for example, by known methods such as CsCl₂ density gradient centrifugation or column chromatography. The RNA is preferably isolated from a cell or a population of cells or from tissue. Where appropriate, the mRNA can be enriched from the RNA, for example by means of chromatrography through an oligo (dT) column.

[0015] The method employs a first primer and a second primer. The first primer is the primer which is used for the reverse transcription. The first primer can also be designated the 3′ primer since it hybridizes with the mRNA and defines the 3′ end of the first strand of the complementary DNA (cDNA). The second primer is used to synthesize the second strand of the cDNA. The second primer is also designated the 5′ primer since it hybridizes with the first strand of the cDNA and defines the 5′ end of the second strand of the cDNA.

[0016] Where appropriate, the first primer and the second primer are labeled with a dye. The first primer is used for the reverse transcription (procedural step a)) and for the subsequent amplification of the cDNA (procedural step c)). The first primer, which is used for the RT, is either labeled with a dye or not labeled with a dye; the latter is, for example, the case when the reverse transcriptase employed does not accept the labeled primer. The first primer is in any case labeled when it is used for amplifying the cDNA. The first primer which is used for the RT and the first primer which is used for the amplification preferably have the same sequence.

[0017] The second primer, which is used both for synthesizing the second strand of the cDNA (procedural step b)) and for amplifying the cDNA (procedural step c)), is preferably labeled both during the second strand synthesis and during the amplification. The second primer preferably has the same sequence on both occasions. The second strand synthesis and the amplification of the cDNA are preferably elements of one reaction such that the second primer is identical.

[0018] In preferred embodiments of the invention, the first and second primers are oligodeoxynucleotides which are composed of deoxyadenosine (dA), deoxyguanosine (dG), deoxyinosine (dI), deoxyuridine (dU), (deoxy)thymidine (dT) and deoxycytidine (dC).

[0019] The first and/or the second primer can have a base sequence (sequence composed of adenine, guanine, cytosine and thymine; termed “sequence” in that which follows) which is complementary to the sequence(s) of one (or more) particular nucleic acid(s). A primer which is complementary to a particular sequence can hybridize with a nucleic acid which contains this complementary sequence or a sequence which is derived from this complementary sequence (that is exhibits a certain homology with this complementary sequence). For example, the first primer can preferably hybridize with the mRNA. The first strand of the cDNA is generated by primer extension. A primer can also have a sequence which is identical, or essentially identical, to the sequence of a particular nucleic acid. This primer can then hybridize (under specific reaction conditions) with a nucleic acid which has a sequence which is complementary to that of the particular nucleic acid. For example, the sequence of the second primer is derived from the sequence of the mRNA, i.e. the sequence of the second primer can be identical or essentially identical to the mRNA sequence. The second primer can therefore (under specific reaction conditions) hybridize with a nucleic acid which possesses a sequence which is complementary to the sequence of the mRNA, e.g. the first strand of the cDNA.

[0020] The reaction conditions under which the first primer and the second primer hybridize with a nucleic acid are preferably specific temperature and/or buffer conditions. The temperature conditions are preferably chosen such that the primers hybridize specifically with the complementary nucleic acid sequence, i.e. such that it is predominantly “correct” base pairings (A-T; G-C) which take place, with as few incorrect base pairings as possible, i.e., if possible, an “optimal” hybridization temperature (annealing temperature) is chosen (e.g. Sambrook et al. (1989) Cold Spring Harbor Laboratory Press). In addition to this, specific salt concentrations, in particular Mg²⁺ concentrations, are chosen.

[0021] A nucleic acid from whose sequence the sequence(s) of the first and/or second primer is/are derived (i.e. to which this/these sequence(s) is/are complementary or identical or essentially identical) can, for example, be a DNA, e.g. a cDNA, a gene, or a part thereof, or an RNA, preferably an mRNA.

[0022] The second primer can possess a random base sequence, with this encompassing both primers which possess a completely random sequence and primers which possess a partially random sequence (e.g. random in relation to one or more bases). While the sequences may be random, they may at the same time exhibit a particular ratio between the individual bases, e.g. all the bases may occur in the same ratio or one or more bases may be over-represented or under-represented. In particular, hypoxanthine can also be used.

[0023] The second primer can also possess one or more defined base sequences. This/these defined sequence(s) may be selected randomly, i.e. they do not derive from known sequences. Primers of this nature can be used to identify new genes or proteins which have not previously been identified.

[0024] The second primer can possess a base sequence which is derived from known sequences, i.e. which corresponds entirely or partially to known sequences (i.e. which is complementary or identical or essentially identical to these sequences). For example, a primer may possess a sequence which exhibits a greater or lesser degree of congruence with a particular consensus sequence or else corresponds to this sequence. This approach can be used, for example, to identify differential expression of known and/or unknown members of a particular gene family/protein family.

[0025] The second primer can possess a base sequence which is derived from a particular amino acid sequence (e.g. a consensus sequence at the amino acid level). In this case, the primers can be degenerate, i.e. allowance is made for the degeneracy of the genetic code in relation to a particular amino acid sequence; the primer then constitutes a mixture of primer molecules which possess different sequences and allow for all the codon usage possibilities for encoding the corresponding amino acid sequence (degenerate primer).

[0026] The sequence of the second primer preferably possesses a restriction cleavage site for a restriction endonuclease.

[0027] The second primer can preferably have a length of from 8 to 20 nucleotides in order to ensure that hybridization is as specific as possible. However, the length is preferably selected in accordance with the particular primer sequences and/or the reaction conditions. The lengths and sequences of the first and second primer are chosen independently of each other.

[0028] The first and/or second primer(s) can also contain (a) sequence(s) which is/are not required for the hybridization and/or which does/do not contribute to the hybridization. Other sequences of this nature can, for example, make possible or facilitate the further characterization or use of the amplified cDNA. These other sequences are preferably located at the 5′ end of the primer. For example, the primer may contain a sequence which facilitates subsequent sequencing e.g. an M13 sequence, and/or a sequence which facilitates the preparation of a labeled probe (e.g. for hybridizing Northern blots or Southern blots and/or for hybridizing in situ), e.g. a T7 promoter sequence, a T3 promoter sequence or an SP6 promoter sequence.

[0029] In the first procedural step, the mRNA is reverse-transcribed, thereby preparing the first strand of a cDNA (procedural step a)). A first primer, which is labeled with a dye, where appropriate, is used for the RT. The first primer preferably possesses an oligo(dT) sequence (“(T)_(X)”, where “X” indicates the number of thymidine residues) which hybridizes with the poly (A) tail of each of the mRNAs and is in this way able to provide a free 3′-OH end for the polymerase reaction. The oligo (dT) sequence is preferably a sequence composed of 10-20 thymidine residues (X=10-20), with a sequence composed of 12 thymidine residues (X=12) or 15 thymidine residues (X=15) being particularly preferred.

[0030] In a particular preferred embodiment, the sequence of the first primer possesses, at the 3′ end of the oligo (dT) sequence, at least one further nucleotide, preferably, however, two nucleotides, which does/do not belong to the oligo (dT) sequence, i.e. denoting that the first nucleotide which is joined on to the 3′ end of the oligo (dT) sequence is different from thymidine. The first primer preferably constitutes a mixture of primer molecules which differ in the sequence of the nucleotides which do not belong to the oligo (dT) sequence. For example, the first primer can have the sequence 5′-(T)_(X)MN-3′, where “M” is A (the base adenine), C (the base cytosine) or G (the base guanine) and “N” is A, C, G or T (the base thymine). For example, X can=12 or 15:

[0031] SEQ ID NO. 1: 5′-TTTTTTTTTTTTMN-3′ (5′-(T)₁₂-MN-3′″),

[0032] SEQ ID NO. 2: 5′-TTTTTTTTTTTTTTTMN-3′ (5′-(T)₁₅-MN-3′″),

[0033] where

[0034] “M” is A, C or G, and

[0035] “N” is A, C, G or T.

[0036] The first primer preferably constitutes a mixture of different primers which differ from each other in the sequence which does not belong to the oligo (dT) sequence.

[0037] 5′-T_((X))MN-3′ can be a mixture of primer molecules having the sequences:

[0038] 5′-T_((X))AN-3′

[0039] 5′-T_((X))CN-3′

[0040] 5′-T_((X))GN-3′

[0041] in which N=A, C, G, T

[0042] 5′-T_((X))MN-3′ is preferably a mixture of primer molecules having the sequences:

[0043] 5′-T_((X))MA-3′

[0044] 5′-T_((X))MC-3′

[0045] 5′-T_((X))MG-3′

[0046] 5′-T_((X))MT-3′

[0047] in which M=A, C, G,

[0048] where the possible bases A, C and G are represented uniformly in the mixture; for example, 5′-(T)_(X)MN-3′ is a mixture of primer molecules having the following sequences: 5′-(T)_(X)AG, 5′-(T)_(X) CG, 5′-(T)_(X) GC, 5′-(T)_(X) AT, 5′-(T)_(X) CT, 5′-(T)_(X) GT, etc. It is possible in this way to provide primers for amplifying different mRNAs of unknown sequence. At the same time, this arrangement ensures that the primers preferentially hybridize with the 3′ end of the complementary sequence (at the poly (A) tail of the mRNA).

[0049] An RNA-dependent DNA polymerase, for example the enzyme reverse transcriptase or another DNA-dependent polymerase having reverse transcriptase activity, e.g. an appropriate temperature-stable polymerase which can then also be used for subsequently amplifying the cDNA (procedural step c)) is preferably used for the RT. In a particular embodiment of the invention, the synthesis of the second strand of the cDNA is an element of the procedural step for amplifying the cDNA, with the procedural step for amplifying the cDNA preferably being a PCR reaction.

[0050] The reverse transcription can, for example, be carried out at 37° C.-50° C., preferably at 40° C.-45° C., particularly preferably at 42° C. Preference is given to using a hybridization temperature which enables the first primer to hybridize specifically (as few incorrect base pairings as possible), which ensures that the activity of the polymerase is sufficiently high and which makes it possible to obtain transcripts which are as complete as possible.

[0051] The second strand of the cDNA is synthesized using a second primer which is preferably labeled. The second primer is preferably at least 6 nucleotides in length (a 6mer), particularly preferably from 10 (a 10mer) to 20 (a 20mer) nucleotides in length. In a particular embodiment of the invention, the second primer is 13 nucleotides in length (a “13mer”). For example, the sequence of the second primer can be “(N)_(X)”, where “X” is 6-20 and where “N” is A, C, G or T, independently of each other.

[0052] The first and second primers are preferably synthetic oligonucleotides which can, for example, be obtained commercially or can be synthesized on a solid phase using a known method (e.g. the phosphoramidite method in accordance with Caruthers et al. (1983) Tetrahedron Letters 24, 245). The primers are preferably composed of the nucleotides deoxyadenosine, deoxyguanosine, deoxyinosine, deoxycytidine and deoxythymidine.

[0053] In a particularly preferred embodiment of the method, the synthesis of the second strand of the cDNAs is already an element of the PCR reaction, with this strand being synthesized under the reaction conditions under which the PCR is carried out.

[0054] The first and second primers are preferably labeled differently. In principle, any type of labelling can be used, for example a primer can be coupled to digoxigenin or biotin such that a cDNA molecule which has been amplified using this primer can be detected, e.g. with an appropriate antibody or enzyme, or a primer can be coupled to a chemical compound which enables a substrate to be converted after it has been added, e.g. Atto-Phos system (JBL Scientific, San Luis Obispo, Calif., USA) or ECF substrate (Amersham-Pharmacia Biotech, Freiburg, Germany).

[0055] However, particular preference is given to labelling the first and/or second primer with fluorescences. Preference is given to labelling the first primer with a first fluorescence and the second primer with a second fluorescence. The first and second fluorescences are preferably chosen such that the two fluorescences employed can be clearly differentiated. In this context, the unambiguity of the result, i.e. the detectable difference between the labelling patterns of individual cDNAs, depends both on the fluorescences employed and on the sensitivity of the analytical method. As a rule, computer-assisted analytical methods are used for evaluating the results. For example, the fluorescences can be selected such that the detectable excitation wavelengths and/or emission wavelengths of the first and second fluorescences differ by 200 nm or more, e.g. 250 nm, 300 nm, 350 nm or 400 nm.

[0056] Examples of fluorescences to which the first and/or second primer can be coupled are Cy2, Cy3, Cy5, FAM, 6-FAM (6-carboxyfluorescein (blue)), FITC (fluorescein isothiocyanate), fluorescein, HEX (4, 5, 2′,4′,5′,7′-hexachloro-6-carboxyfluorescein (green)), 5-IAF, TAMRA (6-carboxytetramethylrhodamine (yellow)), TET (4, 7, 2′, 7′-tetrachloro6-carboxyfluorescein), XRITC (rhodamine-X-isothiocyanate), ROX (6-carboxyrhodamine (red)), Alexa488, Alexa532, Alexa546, Alexa594, Texas red and lissamine.

[0057] For example, the following combinations are possible:

[0058] Primer 1 labeled with Cy5 and primer 2 labeled with Alexa488; primer 1 labeled with Cy5 and primer, 2 labeled with FITC; primer 1 labeled with Cy5 and primer 2 labeled with FAM; primer 1 labeled with Cy5 and primer 2 labeled with Cy2; primer 1 labeled with Cy5 and primer 2 labeled with Cy3; primer 1 labeled with fluorescein and primer 2 labeled with Texas red; primer 1 labeled with fluorescein and primer 2 labeled with lissamine; primer 1 labeled with fluorescein and primer 2 labeled with ROX; primer 1 labeled with fluorescein-Cy3; primer 1 labeled with Alexa594 and primer 2 labeled with Alexa488; primer 1 labeled with Alexa568 and primer 2 labeled with Alexa488; primer 1 labeled with Alexa546 and primer 2 labeled with Alexa488; primer 1 labeled with Alexa532 and primer 2 labeled with Alexa488; primer 1 labeled with Texas red and primer 2 labeled with Alexa488; primer 1 labeled with ROX and primer 2 labeled with Alexa488; primer 1 labeled with Alexa488 and primer 2 labeled with lissamine; primer 1 labeled with Alexa488 and primer 2 labeled with Cy3; primer 1 labeled with Alexa488 and primer 2 labeled with ROX. Naturally, corresponding primer combinations, in which the fluorescences used for the labelling are transposed, with regard to primer 1 and primer 2, as compared with the above examples, are also possible.

[0059] The fluorescences are preferably coupled to the 5′ end of the primer (oligonucleotide). In the case of an oligo (dT) primer, a further nucleotide which is different from thymidine, for example a guanosine, can be located at the 5′ end in front of the fluorescence. In addition, a fluorescence can also be coupled to the oligonucleotide by way of a base. Where appropriate, a fluorescence can also be coupled to the first and/or second primer by way of a suitable linker.

[0060] Examples of labeled primers which can be used as the first and/or second primer are:

[0061] 5′-CY5-G(T)₁₅MN-3′, 5′-CY2-G(T)₁₅MN-3′, 5′-CY3-G(T)₁₅MN-3′,

[0062] 5′-FAM-G(T)₁₅MN-3′, 5′-6-FAM-G(T)₁₅MN-3′, 5′-FITC-G(T)₁₅MN-3′,

[0063] 5′-fluorescein-G(T)₁₅MN-3′, 5′-HEX-G(T)₁₅MN-3′, 5′-5-IAF-G(T)₁₅MN-3′,

[0064] 5′-TAMRA-G(T)₁₅MN-3′, 5′-TET-G(T)₁₅MN-3′, 5′-XRITC-G(T)₁₅MN-3′,

[0065] 5′-ROX-G(T)₁₅MN-3′, 5′-Alexa488-G(T)₁₅MN-3′, 5′-Alexa532-G(T)₁₅MN-3′,

[0066] 5′-Alexa546-G(T)₁₅MN-3′, 5′-Alexa594-G(T)₁₅MN-3′,

[0067] 5′-Texas red-G(T)₁₅MN-3′, 5′-lissamine-G(T)₁₅MN-3′ (the primers which have so far been mentioned are preferably used as first primers), 5′-CY5-(N)₁₃-3′,

[0068] 5′-CY2-(N)₁₃-3′, 5′-CY3-(N)₁₃-3′, 5′-FAM-(N)₁₃-3′, 5′-6-FAM-(N)₁₃-3′,

[0069] 5′-FITC-(N)₁₃-3′, 5′-fluorescein-(N)₁₃-3′, 5′-HEX-(N)₁₃-3′,

[0070] 5′-5-IAF-(N)₁₃-3′, 5′-TAMRA-(N)₁₃-3′, 5′-TET-(N)₁₃-3′,

[0071] 5′-XRITC-(N)₁₃-3′, 5′-ROX-(N)₁₃-3′, 5′-Alexa488-(N)₁₃-3′,

[0072] 5′-Alexa532-(N)₁₃-3′, 5′-Alexa546-(N)₁₃-3′, 5′-Alexa594-(N)₁₃-3′,

[0073] 5′-Texas red-(N)₁₃-3′and 5′-lissamine-(N)₁₃-3′(these primers are preferably used as second primers),

[0074] where

[0075] “M” is A, C or G and

[0076] “N” is A, C, G or T.

[0077] When the first and/or second primer is/are labeled with a fluorescence, care must be taken to ensure that the excitation wavelengths are sufficiently far apart in order to be able to establish, after the excitation, to which wavelength an observable emission was due. An example of a good combination is a Cy5 labelling (excitation at 650 nm), e.g. of the first primer (e.g. of a TX-MN primer), and a fluorescein-based labelling (exitation at 490 nm) of the second primer. The use of two differently labeled primers makes it possible to draw an unambiguous conclusion about the primer composition of (a) detected amplified cDNA(s), e.g. in a gel.

[0078] If, for example, only an emission at 650 nm can be observed, the cDNA was then amplified using the Cy5-labeled primer and this cDNA has the first primer (e.g. the (T)_(X)-MN primer) at one end at least. If an emission can be observed at an excitation wavelength of 490 nm, at least one end of the cDNA is provided with the second primer. If a particular amplified cDNA (in the gel) only emits at one excitation wavelength, then the cDNA was only amplified, in the amplification, using one primer, specifically the primer having a fluorescence which emits at the corresponding excitation wavelength. If a particular amplified cDNA (in the gel) emits at two excitation wavelengths, this cDNA was then amplified using the first and/or the second primer.

[0079] The amplification (preferably PCR) is carried out using a DNA polymerase, preferably a DNA-dependent DNA polymerase, with particular preference being given to this DNA polymerase being a temperature-stable DNA polymerase, e.g. Taq polymerase, VENT polymerase, AmpliTaq polymerase or AmpliTaq Gold polymerase, inter alia. The PCR is carried out using a temperature profile which preferably enables the cDNA to be amplified exponentially. For example, the temperature cycle quoted in Example 1 can be used for this purpose. Preference is given to running 30-40 or more cycles.

[0080] After that, the labeled and amplified cDNA is analyzed. For example, the amplified cDNA can be fractionated in a gel, with amplified cDNAs which differ in their length being located in different bands. The further analysis of these bands can take place, for example, in scanners which excite the fluorescence at a given wavelength (which depends on the fluorescence employed in each case) and/or which are able to detect the light which is emitted in association with the chemical transformation of a chemiluminescent substrate. The emitted fluorescent light can then, for example, be evaluated with an appropriate computer program and assembled into a gel picture (as is known, for example, from autoradiographs). Examples of scanners which can be used are the Fluorlmager 575, Fluorimager SI, Fluorlmager (Molecular Dynamics, Krefeld, Germany), Strom (Molecular Dynamics), FLA-2000 (Fuji, Tokyo, Japan), FMBioll (Hitachi, through Biozym Diagnostic GmbH, Hess, Oldendorf, Germany), Fluor-S-Multilmager and Molecular Imager FX (BioRad, Munich, Germany) scanners.

[0081] The method employs, in particular, mRNA (also termed “mRNA sample”) which is isolated from a cell. As a rule, a sample of this nature is a heterogeneous mRNA sample (i.e. it contains a mixture of the mRNAs which were present in this cell at the time the RNA was isolated). As a rule, this heterogeneous mRNA sample represents the genes which were expressed by a particular cell at a particular time and under particular conditions, i.e. the composition of the mRNA sample depends, for example, on the cell type, its state of differentiation, the cell cycle and/or the previous treatment of the cell, etc.

[0082] One particular embodiment of the invention relates to the analysis of a heterogeneous mRNA sample, which comprises

[0083] a) using a first primer, which is, where appropriate, labeled with a first dye, to prepare the first strand of a complementary heterogeneous DNA sample (heterogeneous cDNA sample) from a heterogeneous mRNA sample,

[0084] b) using a second primer, which is preferably labeled with a second dye, to prepare the second strand of this heterogeneous cDNA sample,

[0085] c) using the first primer, which is labeled with a first dye (“labeled first primer”), and the second primer, which is labeled with a second dye (“labeled second primer”), to amplify the heterogeneous cDNA sample, and

[0086] d) analyzing the composition of the heterogeneous, amplified and labeled cDNA sample.

[0087] The genes which are expressed by a particular cell at a particular time which are especially of interest are those which are differentially expressed or regulated. In the present case, it is in particular the analytical comparison of differentially expressed genes which is of interest. One particular embodiment of the invention relates to a correspondingly analogous method for analytically comparing two or more heterogeneous mRNA samples.

[0088] One particular embodiment of the invention relates to a method for analytically comparing a first heterogeneous mRNA sample with one or more additional heterogeneous mRNA samples, which comprises

[0089] a) using a first primer, which is, where appropriate, labeled with a first dye, for each of two or more heterogeneous mRNA samples, to prepare, in each case, the first strand of a complementary, heterogeneous DNA sample (heterogeneous cDNA sample) from these mRNA samples,

[0090] b) then using a second primer, which is preferably labeled with a second dye, to prepare, in each case, the second strand of the heterogeneous cDNA sample for each of these samples,

[0091] c) using the first labeled primer and the second labeled primer to amplify each of the heterogeneous cDNA samples, and

[0092] d) analyzing the composition of each heterogeneous, amplified and labeled cDNA sample and comparing the compositions of the samples.

[0093] Another particular embodiment of the invention relates to a method which comprises

[0094] a) using a first primer, which is, where appropriate, labeled with a first dye, for each of two or more heterogeneous mRNA samples, to prepare, in each case, the first strand of a complementary, heterogeneous DNA sample (heterogeneous cDNA sample) from these mRNA samples,

[0095] b) using a second primer, which is preferably labeled with a second dye, to prepare, in each case, the second strand of the heterogeneous cDNA sample for each of the samples,

[0096] c) using the first labeled primer and the second labeled primer to amplify each of the heterogeneous cDNA samples, and

[0097] d) carrying out an analysis to determine which of the samples does/do contain or not contain individual mRNA molecules.

[0098] After the cDNA has been amplified and the composition of the cDNA has, where appropriate, been analyzed, an investigation is carried out to determine which of the amplified cDNAs have which primer composition, i.e. which cDNAs were amplified using only the first primer, which were amplified using only the second primer and which were amplified using the first and the second primers (affiliation of the differentially amplified cDNAs to groups 1)+2), 3) and 4)). The invention also relates to follow-up methods in which particular amplified cDNAs are selected on the basis of the primer composition of the amplified cDNAs and, where appropriate, subjected to further analysis and/or further use. In addition to this, the invention relates to follow-up methods in which the distribution of the primer composition of the totality of the amplified cDNAs is analyzed with regard to groups 1)-4).

[0099] Examples of follow-up methods are:

[0100] A method for analyzing an RNA sample, preferably an mRNA sample, which comprises

[0101] a) using a first primer, which is, where appropriate, labeled with a first dye, to prepare the first strand of a complementary DNA sample (cDNA sample) from an RNA sample, preferably an mRNA sample,

[0102] b) using a second primer, which is preferably labeled with a second dye, to prepare the second strand of this cDNA sample,

[0103] c) using the first primer, which is labeled with a first dye, and the second primer, which is labeled with a second dye, to amplify the cDNA sample, and

[0104] d) analyzing the composition of the amplified and labeled cDNA sample, and

[0105] e) determining the primer composition of amplified cDNAs.

[0106] A method for analyzing an RNA sample, preferably an mRNA sample, which comprises

[0107] a) using a first primer, which is, where appropriate, labeled with a first dye, to prepare the first strand of a complementary DNA sample (cDNA sample) from an RNA sample, preferably an mRNA sample,

[0108] b) using a second primer, which is preferably labeled with a second dye, to prepare the second strand of this cDNA sample,

[0109] c) using the first primer, which is labeled with a first dye, and the second primer, which is labeled with a second dye, to amplify the cDNA sample, and

[0110] d) analyzing the composition of the amplified and labeled cDNA sample,

[0111] e) determining the primer composition of amplified cDNAs, and

[0112] f) selecting cDNAs having a particular primer composition, preferably those cDNAs which contain both the first and the second primer, and subjecting them to further analysis.

[0113] The RNA or mRNA samples which are used for the comparative analysis or methods can, for example, be isolated from different cells whose expression patterns are to be compared with each other. In this context, the mRNA samples can, for example, be mRNA samples which are derived from different cells or cell types which differ from each other in their stage of differentiation and/or development or in the stage of the cell cycle or which have a different history (e.g. different culture conditions (e.g. pH, temperature or composition), or which have been treated with pharmacological active compounds or disease-promoting/disease-inhibiting or disease-inducing substances (e.g. carcinogenic or mutagenic substances) or have been exposed thereto (e.g. UV light)). Such RNA or mRNA samples can be compared with RNA or mRNA samples which have not been exposed, or not exposed to this extent, to these substances. The RNA or mRNA samples can be isolated from particular tissues. For example, healthy tissue can be compared analytically with pathological tissue, young tissue with old tissue, treated tissue with untreated tissue, and induced tissue with non-induced tissue, while tissues from different stages of development and/or the cell cycle and/or differentiation can also be compared analytically, with tissue denoting tissue, organ, cell type, culture-derived cells, cells of a cell line or the cells of an individual.

[0114] The method can be used for analyzing differential gene expression (in a tissue or in a cell). In particular, the method can be used for analytically comparing differential gene expression (in two or more tissues or cells).

[0115] The method can be used for identifying and/or characterizing pharmacological active compounds. Furthermore, the method can be used for identifying and/or characterizing target genes or target proteins. These target genes or target proteins are, in particular, genes or proteins which have a function (which is as specific as possible) in the prevention, origin and/or progress and/or healing of a disease, in the differentiation process, e.g. cell differentiation (where appropriate, before or after induction) and/or in the differentiation process of a disease, in the cell cycle or cell cycle control and/or cell development, e.g. the process of cell aging.

[0116] The method is seen to be advantageous in comparison with conventional radioactive DDRT since the use of fluorescence-labeled primers can be substituted for the use of radioactively labeled nucleotides or radioactively labeled primers. In the novel method (DDRT variant) which is described here, in contrast to the known fluorescence DDRTs, each amplified cDNA is labeled by using two primers which are differentially labeled, preferably with fluorescences having different excitation spectra. In this way, by using an appropriate evaluation method, it is possible to detect each amplified, labeled cDNA, i.e. the complexity of the amplified cDNA is retained and is detectable (products 1), 2), 3), and 4) can be detected; e.g. the band complexity in the gel is retained). In addition to this, it is possible to draw unambiguous conclusions about the primer composition of an amplified labeled cDNA (i.e. it is possible to assign the cDNA to 1), 2), 3) and 4)). This makes it possible to select the cDNAs which are to be subjected to further analysis because of their primer composition, e.g. to select amplified cDNAs which actually do derive from the 3′ regions of the gene sequences. When this method, which is based on using two differently labeled primers, is employed, it is possible, therefore, to reduce the time and expense involved in possibly having to isolate and analyze redundant cDNAs. Despite this, all the amplified cDNAs are detected, and can be analyzed, simultaneously, as in conventional radioactive DDRT, which is a feature which is not afforded in conventional fluorescence DDRT.

[0117] The invention also relates to a test kit for implementing the method.

EXAMPLES

[0118] The enzymes employed are obtained from Gibco BRL/Life Technologies (Karlsruhe, Germany) (reverse transcriptase and Taq polymerase) and Promega (Heidelberg) (RNAsin).

[0119] Thermocycler: Perkin Elmer GeneAmp PCR System 2400 (Perkin Elmer, Weiterstadt).

Example 1

[0120] Reverse Transcription

[0121] Reaction mixture: 1 μl of RNA (100 ng-1 μg of total RNA or 1 ng-10 ng of poly-A-RNA/mRNA), 1 μl of primer 1 (10 μM primer 1, e.g. (T)_(X)-MA-3′ or Cy5-(T)_(X)MA-3′, where M=A, C, G and X=11-15), and 8 μl of H₂O (nuclease-free). The reaction mixture was incubated at 70° C. for 5 min and then placed on ice.

[0122] 4 μl of 5-times RT buffer (Gibco BRL), 2 μl of DTT [0.1M], 1 μl of SuperScript reverse transcriptase [200 U/μl] (Gibco BRL), 1 μl of RNAsin [40 U/μl] and 2 μl of dNTP [250 μM] were then added.

[0123] The reverse transcription was carried out at 37°-50° C. for 60 min and the enzyme was then inactivated at 70° C. (10 min).

Example 2

[0124] PCR

[0125] 2 μl of 10-times PCR buffer (Gibco BRL), 0.9 μl of W-1 detergent [1%] (Gibco BRL), 0.75 μl of MgCl₂ [50 mM] (Gibco BRL), 1.6 μl of dNTP [250 μM], 0.5 μl of Taq DNA polymerase [5 U/μl] (Gibco BRL), 1 μl each of primer 1 and primer 2 ([10μM]; e.g. Cy5-T7-(T)_(X)-MA-3′ as primer 1 (where “T7” is a sequence segment from the T7 promoter) and fluorescein-(N)_(X), where X=10-25 and N=A, C, G, T) and 10.25 μl of H₂O were added to 2 μl of the RT mixture from Example 1.

[0126] The PCR was carried out under the following conditions: 5 min at 94° C. 40 ×: 1 min at 94° C., 2 min at 40° C.-60° C., 1 min at 72° C. 7 min at 72° C.

[0127] After the conclusion of the PCR, the reaction mixture is stored at 4° C. and then fractionated on a sequencing gel (5% polyacrylamide; 8 M urea).

[0128] The labeled amplified cDNAs can be detected, for example, using a scanner which excites at a wavelength of 430+/−30 nm (for fluorescein: excitation wavelength 490 nm, emission wavelength 520 nm) and 635+/−5 nm (for Cy5: excitation wavelength 650 nm, emission wavelength 675 nm). The Molecular Dynamics Storm Imager is an example of this type of scanner.

1 2 1 14 DNA Artificial Sequence exon (1)..(14) Description of Artificial Sequence synthetic “V=A,C,G; N=A,C,G,T” 1 tttttttttt ttvn 14 2 17 DNA Artificial Sequence exon (1)..(17) Description of Artificial Sequence synthetic “V=A,C,G; N=A,C,G,T” 2 tttttttttt tttttvn 17 

1. A method for analyzing an RNA sample, which comprises a) using a first primer, which is, where appropriate, labeled with a first dye, to prepare the first strand of a complementary DNA sample (cDNA sample) from an RNA sample, b) using a second primer, which is preferably labeled with a second dye, to prepare the second strand of this cDNA sample, c) using the first primer, which is labeled with a first dye, and the second primer, which is labeled with a second dye, to amplify the cDNA sample, and d) analyzing the composition of the amplified, labeled cDNA sample.
 2. The method as claimed in claim 1, wherein the first primer contains an oligo (dT) sequence.
 3. The method as claimed in one or both of claims 1 and 2, wherein the oligo (dT) sequence of the first primer is composed of at least 10 thymidine nucleotides.
 4. The method as claimed in one or more of claims 1 to 3, wherein the first primer contains, at the 3′ end of the oligo (dT) sequence, at least two further nucleotides, which do not belong to the oligo (dT) sequence.
 5. The method as claimed in one or more of claims 1 to 4, wherein the first primer is a heterogeneous mixture of primer molecules which differ in the sequence of the nucleotides which are in positions M or N and which do not belong to the oligo (dT) sequence.
 6. The method as claimed in one or more of claims 1 to 5, wherein the second primer is at least 6 nucleotides in length.
 7. The method as claimed in one or more of claims 1 to 6, wherein the first and second dyes are different fluorescences.
 8. The method as claimed in one or more of claims 1 to 7, wherein the fluorescences are selected from the fluorescences Cy2, Cy3, Cy5, FAM, 6-FAM, FITC, fluorescein, HEX, 5-IAF, TAMRA, TET, XRITC, ROX, Alexa488, Alexa532, Alexa546, Alexa594, Texas red and lissamine.
 9. The method as claimed in one or more of claims 1 to 8, wherein, apart from a first RNA sample, one or more additional RNA samples are analogously compared analytically.
 10. The use of a method as claimed in one or more of claims 1 to 9 for analyzing differential gene expression.
 11. The use of a method as claimed in one or more of claims 1 to 9 for identifying and/or characterizing pharmacological active compounds.
 12. The use of a method as claimed in one or more of claims 1 to 9 for identifying target genes. 